Wireless cellular communication networks and their operation are generally well known. In such a system the area covered by the network is divided into cells. Each cell is provided with a base station, which is arranged to communicate with a plurality of mobile stations or other user equipment in a cell associated with the base station.
In these known systems, it is possible to locate a mobile station with reference to a base station, and therefore possible to locate a mobile station within the operational transmission range of a base station.
As is also known additional location information can be determined by measuring the time between transmission and reception of a signal between a mobile station and a known base station or transmitter. Using such time of arrival (TOA) methods with signals transmitted from base stations it is possible to locate a mobile station within tens of metres.
Using the base station to transmit timing signals and using these signals to determine a location estimate produces an estimate containing several potential errors and problems One of the major problems is the many different paths that the transmissions from the base station to the mobile station can take. The path can be direct, which provides an accurate estimation of the distance between the base and mobile stations or the path can be diffracted or reflected by man-made or natural phenomena such as buildings, large vehicles and hills. These indirect paths do not reflect the true distance between the base station and the mobile station and therefore produce location estimation errors. These diffracted and reflected signal paths occur more frequently in built-up and urban environments, thus degrading the more accurate base station location estimations due to the increased density of base stations.
A separate development in location estimation has been the development of a global positioning satellite (GPS) system which enables a GPS receiver to accurately locate its location within a couple of metres by measuring the time differences between received signals from satellites orbiting the earth. The GPS system relies on both the transmitter (the orbiting satellites) and the receiver to have accurate knowledge of a transmitted timing sequence signal in order that an accurate estimation of the location of the receiver can be made.
As is known in the art the GPS orbiting satellites are accurately synchronised each carrying an accurate very stable atomic clock. Furthermore the constellation of satellites is monitored from controlling ground stations and any timing errors detected are effectively corrected.
As the cost of supplying each GPS receiver with an accurate and stable clock oscillator such as an atomic clock is prohibitive, the typical GPS receiver determines an accurate GPS time sequence by comparing at least four separate GPS timing signals received from at least four different satellites. These satellites are used to both accurately synchronise the receiver clock and to provide an accurate estimation of the location of the signal.
As it is known in the art a timing synchronisation sequence can be carried out by receiving the Time of Week (ToW) signal transmitted by each GPS satellite. The ToW signal is transmitted once per GPS subframe, in other words exactly every six seconds. The detection of the ToW signal is largely dependent on the received strength of the signal, and below a certain threshold it becomes impossible to decode the information bits that go to make up the ToW signal. Additionally, processing the ToW signal takes up a significant amount of processing time which has an adverse impact on power consumption.
The speed of producing an a timing synchronisation sequence (and also therefore location estimation) where the received signal is close to the received strength threshold can be improved in some situations by storing a previously determined location estimate and using this estimate as an a priori value to limit the ‘search window’ for the timing synchronisation sequence.
These methods rely on time stamping the stored location estimates, and discarding location estimates older than a predefined value. This prevents the timing synchronisation sequence starting its search from an inaccurate location search window.
These methods have a disadvantage when old stored locations are not inaccurate starting locations. The known methods simply discarding the location estimates and require the system to start a fresh timing synchronisation sequence, with its resultant processing and power costs.